1. Field of the Invention
The present invention relates to a pulp mill recovery system. More specifically, the present invention relates to a low temperature kraft spent liquor recovery system utilizing separate reactors for pyrolysis, combustion and sulfate reduction.
2. Description of the Prior Art
The central piece of equipment for recovery of cooking chemicals and energy from kraft black liquor is the so-called Tomlinson furnace. Black liquor at about 65% dry solids content is sprayed into the furnace. During their descent, the black liquor droplets lose the remaining water by evaporation and the solids pyrolyze to form a char bed at the bottom of the furnace. The char bed burns under reducing conditions at a temperature of about 750.degree.-1050.degree. C. and the recovered chemicals, mainly Na.sub.2 CO.sub.3 and Na.sub.2 S, are drained from the furnace as a smelt. The smelt is dissolved in water to produce so-called green liquor, the precursor of the cooking liquor called white liquor. The gases generated during pyrolysis and burning of the char are fully combusted at a higher location in the furnace. The furnace is provided with suitable heat exchange means to recover heat from the hot combustion gases for steam and electricity generation.
Although the objective of the recovery of chemicals and energy is adequately achieved in present commercial operations, the use of the Tomlinson furnace presents a number of problems. For example, inadvertant contact between water and the inorganic smelt has resulted in serious explosions. Also, high char bed temperatures lead to increasing emission of sodium salts and excessive fouling of the steam pipes in the upper part of the furnace.
To solve these problems, and also to reduce capital investment and increase the energy efficiency of the recovery operation, a number of kraft recovery alternatives have been described. In some of these alternatives the smelt-water explosion hazard is eliminated and the emission of sodium salts reduced by keeping the inorganic chemicals in solid rather than molten form. This principle was used by Copeland et al., U.S. Pat. No. 3,309,262, where spent liquor is concentrated and introduced by atomization into a fluidized bed reactor. The resulting waste liquor spray encounters residual inorganic chemicals derived from the combustion of previous spent liquors. Additionally, the fluidized bed reactor may contain inert materials such as silica grains in admixture with the inorganic chemicals. In the fluidized bed reactor, operated with excess air, all the organic material is combusted below the fusion point of the inorganic salt mixture. The sodium sulfate in the inorganic pellets are reduced with hydrogen in a second fluidized bed (Arnold, Can. Pat. 828,654). Alternatively, the first fluid bed can be used as a means to provide incremental recovery capacity, while the reduction of sodium sulfate is achieved by injecting the pellets into the conventional recovery furnace (Tomlinson II, U.S. Pat. No. 4,011,129).
Flood, U.S. Pat. No. 3,322,492, describes a two-stage fluid bed process where weak black liquor at about 20% solids content is dried to solid granules in the first bed at a temperature of about 175.degree. C. The sodium sulfate in the granules is reduced to sodium sulfide by virtue of carbon monoxide derived from decomposition of the organic matter in the second bed. The operating temperature of the second fluid bed is about 800.degree. C.
Osterman, U.S. Pat. No. 3,523,864, presents a three-zone fluid bed reactor which would replace the conventional chemical recovery furnace and lime kiln. Black liquor is dried and burned under reducing conditions at about 650.degree.-700.degree. C. in the intermediate zone. The reducing gas from the intermediate zone is burned and serves as fluidizing medium for the top fluidized bed. Here predried CaCO.sub.3 is introduced to be calcined to CaO pellets. These CaO pellets overflow first to the intermediate zone and then subsequently to the lower bed with a coating of mainly char, Na.sub.2 SO.sub.4 and Na.sub.2 CO.sub.3 from the burned black liquor. The reduction of Na.sub.2 SO.sub.4 is said to take place in the lower fluidized bed at about 700.degree.-760.degree. C. with air and/or combustion gases as a fluidizing medium.
In the process of Shah, U.S. Pat. No. 3,574,051, kraft black liquor is concentrated by contact with a stream of heated air. The resulting concentrated black liquor is then burned with excess air in a fluidized bed reactor while the bed temperature is maintained at about 250.degree.-600.degree. C. The solid salts are then passed through another reactor and subjected to a reducing gas stream containing mainly carbon monoxide. It is claimed that in the range of 250.degree.-500.degree. C. the sodium sulfate is reduced to sodium sulfide. Green liquor is produced by dissolution of the salts in water.
Lange, Can. patent 1,089,162, presents a low temperature process where the organic portion of black liquor is gasified in a fluidized bed, operating not in excess of 760.degree. C. so as to keep the inorganic portion of black liquor in the solid state. The solid particles leaving the bed will typically contain 90% Na.sub.2 CO.sub.3, 9% Na.sub.2 S, less than 1% Na.sub.2 SO.sub.4, and less than 1% carbon. After dissolving the solids in water, and separation of the carbon, the liquor will be used to remove H.sub.2 S from the gas produced in the fluidized bed reactor. The spent absorbing medium can then be treated to form the cooking liquor which is returned to the digestion process.
In all the above alternatives to the conventional kraft recovery process (except for the process of Tomlinson II, U.S. Pat. No. 4,011,129), Na.sub.2 S and Na.sub.2 CO.sub.3 are produced from black liquor in reactors operating below the fusion point of the inorganic salt mixture. As far as is known, only the Copeland process is used on a commercial scale. However, in this process the end products are pellets consisting of mainly Na.sub.2 SO.sub.4 and Na.sub.2 CO.sub.3 rather than mainly Na.sub.2 S and Na.sub.2 CO.sub.3. There are two main reasons for the absence of commercial utilization of these low temperature processes. First, the relatively high temperature required for fast and complete conversion of Na.sub.2 SO.sub.4 to Na.sub.2 S and, secondly, the ease of formation of H.sub.2 S when Na.sub.2 S is contacted with combustion gases below the melting point of the inorganic salts. So, while the reduction is favored by a high temperature, the above alternative processes require a relatively low temperature just below the melting point of the inorganic salt mixture. The consequence is that in fluid bed processes operating in the reducing mode, most of the formed Na.sub.2 S is rapidly converted to H.sub.2 S (and some COS) according to the overall reaction EQU Na.sub.2 S+CO.sub.2 +H.sub.2 O.fwdarw.Na.sub.2 CO.sub.3 +H.sub.2 S
resulting in a low yield of solid Na.sub.2 S.
It is an object of this invention to provide a kraft recovery process whereby Na.sub.2 CO.sub.3 and Na.sub.2 S are formed below the melting point of the inorganic pulping chemicals with a minimum production of sulfurous gases.
It is a further object of this invention to provide an assembly for carrying out the process, more especially an assembly of reactors.